The present invention relates to expansion valves for controlling flow of refrigerant between the exothermic heat exchanger or condenser and the endothermic heat exchanger or evaporator in a refrigeration or air conditioning system. Valves of this type are employed for creating a sufficient pressure drop and expansion of the refrigerant prior to entry into the evaporator.
One type of refrigerant expansion valve employed in air conditioning and refrigeration systems is a pressure operated temperature sensing device which responds to the temperature of the refrigerant to expand fluid in an enclosed chamber to act upon a diaphragm and control the position of a valve member. Such pressure operated refrigerant expansion valve responds to an existing temperature condition of the refrigerant; and; thus control of the refrigerant flow lags the temperature of the air to be refrigerated or conditioned in the compartment.
In stationary refrigeration or air conditioning systems, the rate of change of ambient conditions is generally slow and the response of the system is more than adequate with a pressure sensitive diaphragm expansion valve because the lag or delay is not significant with respect to the rate of change of ambient conditions.
However, in air conditioning systems employed for the passenger compartment of vehicles, ambient conditions change rapidly. In such applications, a valve which responds only to the temperature of the refrigerant cannot respond in sufficient time or anticipate changed conditions to provide the desired degree of refrigerant flow control necessary to maintain the desired regulation of passenger compartment temperature.
Therefore, it has long been desired to provide an electrically operated refrigerant expansion valve in a vehicle air conditioning system in order that the flow of refrigerant can be controlled in response to parameters other than a simple sensing of refrigerant temperature.
Examples of vehicle air conditioning systems employing electrically operated expansion valves and various electrical strategies for valve operation are shown and described in the above cross-referenced related applications. Where rapid response to changing ambient conditions is required for a vehicle air conditioning system, it has been found desirable to generate an electrical control signal providing for modulation of the electrically operated expansion valve by employing a width-modulated pulse signal of substantially constant frequency, or period and varying pulse width or providing a fractional duty cycle for proportional control of the valve opening and consequently proportional control of the refrigerant flow to the evaporator.
In providing electrically operated expansion valves for vehicle air conditioning systems, it has been found desirable to provide a relatively high frequency electrical control signal to accommodate the rapid changes in the varying load on the system. Where it has been found desirable to use an electrically operated expansion valve for controlling refrigerant flow, problems have been encountered in providing a valve which in the closed position would permit a small or limited amount of flow to the evaporator and yet could be proportionately controlled to provide a controlled amount of flow over a relatively wide range of flow.
A simple poppet type valve has the disadvantage that in the closed position, all flow is blocked and that a small amount of movement of the poppet from the fully closed position produces a sudden surge of flow which is extremely difficult to control at low rate of flow in the nearly closed position. Thus, it has long been desirable to find an electrically operated refrigerant expansion valve which provides good proportional control of flow over the desired range and also in the closed position provides a limited but controlled residual flow. It has further been desired to provide such a valve which is capable of extended use and continuous relatively high frequency cycling for extended periods of time without succumbing to leakage or stiction and also to provide such a valve which can be operated with relatively low power consumption at low voltage.